A repeater system (such as a distributed antenna system (DAS) or a single-node repeater) is typically used to improve the wireless radio frequency (RF) coverage provided by one or more base stations. The wireless service provided by each base station can include commercial cellular service or private or public safety wireless service. The repeater system can be implemented as an “active” system in the sense that the downlink and uplink RF signals are amplified before being repeated.
Active repeater systems can be used, for example, in sport stadiums, buildings (hotels, malls, or trade centers), metro stations and airports, trains, and tunnels. Each base station can be coupled to the repeater system via one or more cables or via a wireless connection, for example, using one or more donor antennas. Repeater systems can be used in other applications.
The capacity of each base station coupled to a repeater system can be dedicated to the repeater system. The capacity of each base station coupled to a repeater system can also be shared among the repeater system and a base station antenna system that is co-located with the base station and/or shared among the repeater system and one or more other repeater systems.
Active repeater systems typically suffer from intermodulation distortion. For example, to obtain higher power added efficiency, power amplifiers in such systems are operated at saturation, that is, in a non-linear region. As a result, two or more radio carriers, e.g., in a downlink band, may be amplitude modulated and create an undesired intermodulation distortion (IMD) product. IMD products can create adjacent channel interference. If IMD product(s) fall within a receive band, sensitivity and dynamic range of the receiver in the base station can be reduced.
If the receiver in the base station is able to receive signals with increased sensitivity, it can receive weaker signals, which can be used to increase the reception range and overall coverage area of the base station and the associated repeater system. This can result in cost savings as fewer base stations and repeater systems (or components thereof such as DAS remote units) may be required to provide the desired wireless coverage and capacity.
Digital pre-distortion (DPD) is used to diminish IMD products at the output of a power amplifier. An exemplary technique for digital pre-distortion is illustrated in US patent publication US20120200355 A1 (the '355 Publication), which is incorporated herein in its entirety. The '355 Publication discloses the use of a coupler to feedback a portion of an output signal from a power amplifier. The output of the coupler is provided to a RF to digital converter which down converts and digitizes the feedback signal to a coefficient estimator. The coefficient estimator supplies coefficients to a digital pre-distortion module that generates tones of appropriate phase and amplitude to diminish IMD products, such as third order IMD products, at the output of the power amplifier.
Digital pre-distortion circuitry adds cost to the systems in which it is used. For broadband DASs and single-node repeaters, two or more power amplifiers may be needed to cover different RF transmit bands. Typically, each power amplifier is preceded by its own DPD circuitry. This undesirably increases system cost.